Key Points

Contracting the rectus abdominis and external obliques during a squat or deadlift results in the erector spinae having to do more work than necessary.

EMG research does not support the assertion that the abdominals activate to a large degree in either the squat or deadlift.

In order to develop the abdominals, perform abdominal exercises.

The postulate that squats and deadlifts are effective abdominal builders has been around for some time. By whom and when this was first purported, I am not sure, but I do know that, biomechanically, it does not make sense.

Background

Back in March of this year, I commented on one of No Bullshit Bodybuilding’s (Ian McCarthy) threads, and that comment (below) was later reposted for further discussion; the response to said comment was divided, with those not in agreement having strong views against what I had said.

“Contracting the abdominals would result in a spinal flexion moment, partly countering of the extension moment produced by the erectors. This would result in a faster fatiguing of the erectors, a smaller net extension moment, unnecessarily larger compression forces, and possibly, less weight lifted. The rectus abdominis is an antagonist of the erector spinae; it would be, to be blunt, stupid to contract it, let alone to a large degree, in a movement that requires such a large extensor moment. The idea that the abdominals work to a large degree in movements like the squat and deadlift is not only unfounded and unsubstantiated, but is also unwarranted, as it does not make sense, biomechanically, to activate them in such a movement.” – Andrew Vigotsky

The background of this comment may be of importance, as it was in regards to abdominal development for physique athletes (i.e., bodybuilders, figure, etc.), so the rectus abdominis, and to a lesser degree, the external obliques. The conversation was sparked by Brad Schoenfeld’s post about the illogicalness of performing high repetition abdominal exercises (below), to which someone responded something along the lines of, “Just squat and deadlift.”

"Would anyone with any knowledge of exercise science do 500 reps a day of leg work to maximize development of their quads? If not, then what's the rationale for doing so for the abs?" - Brad Schoenfeld

However, I understand that not everyone may be familiar with the terms used in my comment, so Ian asked me to break things down and provide more details, to which I happily obliged. For the purpose of this article, “abdominals” refers to the rectus abdominis and external obliques, as these are the abdominals most pertinent to bodybuilding.

Spinal Extension Moment Requisites

Understanding what a moment of force is, and how it impacts our movement, is essential to understanding the complex role of the core during squats and deadlifts. A moment of force is a turning force, and is equal to the magnitude of the force times the perpendicular distance to the joint center (). Moments are analogous to torques, but torques are turning forces along the long axis of something (such as bone), which produces torsion (9). Balancing moments can be thought of as a seesaw, with the force being the weight, and perpendicular distance being the distance from the pivot point.

The seesaw analogy

The seesaw is perhaps the most relatable analogy for understanding what happens in our bodies, and by using a seesaw as a model, we can better understand how and why joints act the way they do.

When squatting or deadlifting, one should maintain what is called static equilibrium of the spine; in other words, the spine should be “balanced”, like a seesaw. Static equilibrium can be achieved in a number of ways. In the case of most joints, including the spine, a muscle is like a fat kid, sitting close to the seesaw’s axis of rotation, and the external load is like a skinny kid, sitting far away from the seesaw’s axis of rotation. The fat kid must weigh enough to make up for skinny kid sitting farther away (below).

The forces are different (left is half that of the right), but because the lighter weight is farther from the pivot point (left is double that of the right), or axis of rotation, the moments are balanced, resulting in a net moment of zero.

If the seesaw were to rotate counterclockwise, it would be representative of spinal flexion, and if it were to rotate clockwise, it would be representative of spinal extension. In a squat or deadlift, the external load would be the load on the left, and the erector spinae force on the right. If we were to include the abdominals, however, the seesaw would change, and more force would be required from the erector spinae (below).

The addition of the abdominal (middle) must be countered by an increase in force output from the erector spinae (right).

Mechanical Explanation

In the sagittal plane, two moments can be applied to the spine: a flexion moment and an extension moment. The former forces your spine to round forward, or flex, and the latter to round backward, or extend. When discussing the squat or deadlift, the load would be on one side, and the erector spinae muscle would be on the other. If the abdominals were to be included, they would be included on the side with the load, because they both try to “tip” the seesaw in the same direction, or try to flex the spine, while the erector spinae tries to extend the spine. Because the load in the squat and deadlift is anterior to the spine, it places a flexion moment on the spine, which must be countered by an extension moment in order to keep the spine in static equilibrium, or prevent it from moving/rounding (1). It should be noted that, although the squat and deadlift are dynamic movements, Lander et al. (5) determined that quasi-static assessments were 99% as effective as inverse dynamics during heavy squats. In the case of static equilibrium, the sum of all moments equals zero (), which means there is a net moment of zero. Should a net moment exist, then an angular acceleration will occur in the direction of that net moment; for example, if there is a net flexion moment, then the person’s spine will accelerate into flexion, proportionally to the magnitude of that net flexion moment.

The Proof Is in the Pudding

Mathematical Relationship

The previous paragraphs explain the mechanical roles of the abdominals, erector spinae, and external load on the spine during a squat and deadlift. In order to better explain this relationship, it is essential to quantify it, while also looking at the data.

The erector spinae has an extension moment arm of about 6.0cm at the L4/L5 level (3).

Both the rectus abdominis and external oblique have flexion moment arms of about 8.2cm (7).

This means that, for each newton produced by the abdominals, the erector spinae must produce about 1.4 N in order to maintain spinal static equilibrium.

Experimental Evidence

It is clear that, theoretically, it does not make sense to contract the abdominals during a squat or deadlift, as it would require substantially greater force output from the erector spinae in order to maintain a neutral spine; however, experimental evidence does not always jive with theory, so it is essential to explore experimental data, too.

Currently, EMG is the best method we have to detect whether or not a muscle is active, and how active that muscle is, which may or may not be indicative of force, and ultimately moment, contributions (depending on that muscle’s position). Working with and interpreting EMG can be complex, so understanding it and its limitations are an imperative prerequisite to its discussion; therefore, for those that have not read the article on EMG that Bret and I wrote last year, and would like a deeper understanding of EMG, I would highly recommend doing so.

Squat

Research on squat abdominal EMG activity is unequivocal, with all studies with proper normalization procedures showing around 10% or less of maximum voluntary isometric contraction (MVIC) in the rectus abdominis, and around 20% or less of MVIC in the external obliques (1, 2, 11). Bressel et al. (2) even had participants consciously contract their abdominals, which did not result in a large increase in abdominal EMG activity. For those interested in seeing the tables and charts from these studies, I have uploaded them to Imgur.

Deadlift

To my knowledge, there are only two studies examining abdominal EMG activity in the deadlift. Investigators of one found that EMG activity of both the rectus abdominis and external obliques was slightly below 60% MVIC (4). There is one caveat, however, and that is that the normalization procedure used by the authors is not the “gold standard”; more specifically, the authors utilized a crunch, while the normalization procedure that elicits the greatest EMG activity tends to involve resistance against the torso, while hanging off a table (6). Having said that, 60% is not an extremely high level of activation; never mind that the normalization procedure was suboptimal. The other study, Willardson (11), which utilized 75% of participants’ 1RM, noted only 15% MVIC in the external obliques, and 4-5% MVIC in the rectus abdominis.

EMG Conclusions

It is abundantly clear that neither the squat nor deadlift elicit a substantial amount of EMG activity in the rectus abdominis or external obliques.

Intra-abdominal Pressure and Other Abdominal Muscles

Intra-abdominal pressure (IAP) is the increase in pressure in the abdominal cavity, which creates an extensor moment, and in turn, may aid the erector spinae. The assertion that high levels of rectus abdominis and external oblique muscle activity are needed to generate maximum IAP, however, is unfounded. McGill et al. (8) found that large IAP can be generated during squat lifts without high activation of the rectus abdominis or external obliques.

My postulation is that IAP is created via the contraction of the diaphragm, which passively stretches the transverse abdominis (as the transverse abdominis is not activated to a high degree, either (11)), which generates IAP, and thus, an extensor moment; however, I have not seen data elucidating this hypothesis.

Addressing Criticism

It pleases me that someone was skeptical of my original comment, to the point where they asked respected experts in the field of powerlifting what their views were on this topic. Mark Rippetoe responded with the following.

I seldom comment on internet comments, but I'll just ask you to think about the role of isometric contraction of abs, erectors, and everything else in a squat or pull. Not every muscle contraction results in a change in muscle belly length. I thought this was commonly-possessed understanding -- agonist/antagonist function, a balance of extension and contraction, etc. This type of thinking is typical of "physios" who only consider the isolated contractile function of individual muscles. What are your comments?

I want to make it perfectly clear that, just because an action is isometric, resulting in no change in length or net moment, does not mean that activating the abdominals would not affect how efficiently the movement can be performed. The system is a sum of its parts (or moments). This is akin to flexing the elbow 90º, such that the forearm is perpendicular with the body, which can be accomplished by a small contraction of the elbow flexors, or a large contraction the elbow flexors and a moderate contraction of the elbow extensors. Both of the aforementioned scenarios would result in the same kinematic outcome, but the former is much more efficient.

Conclusions

The claim that squats and deadlifts are effective abdominal exercises is still unfounded and unwarranted. The biomechanical rationale for such a claim is nonexistent, and here, I have explained why consciously contracting the abdominals would result in a less efficient movement. Bressel et al. (2) noted, “conscious co-activation of the trunk muscles during the squat exercise may lead to spinal instability and hazardous compression forces in the lumbar spine.” The only moment countering the abdominal’s flexion moment is the extension moment produced by the erector spinae, which is already acting to counter the load’s flexion moment. It is recommended that trainees seeking to develop their abdominals utilize abdominal exercises that provide an external extension moment, rather than an internal one; for example, RKC planks (10). Abdominal-specific exercises will require a larger force production from your abdominals, thus resulting in greater activation. Now, let’s end the dogma that squats and deadlifts are good abdominal activators, at least until we have evidence showing they are, because at this point, neither experimental data nor theory jive with the assertion that they are.

9. Paul J. Torques produce torsion. J Biomech 11: 87, 1978.

10. Schoenfeld BJ, Contreras B, Tiryaki-Sonmez G, Willardson JM, and Fontana F. An electromyographic comparison of a modified version of the plank with a long lever and posterior tilt versus the traditional plank exercise. Sports biomechanics / International Society of Biomechanics in Sports 13: 296-306, 2014.